Negotiators for the Metro transit authority and the Washington area's largest transit union announced a tentative agreement yesterday on a new labor contract that includes major changes in the union's controversial cost-of-living benefits.

Carmen E. Turner, the Metro system's recently appointed general manager, praised the proposed agreement as a "new beginning" in labor relations.

The accord, officials noted, was the first to have been reached between Metro and union negotiators in nine years without the intervention of outside arbitrators.

The cost of the tentative settlement--a key issue for the financially strapped Metro system--was in dispute yesterday.

Thomas R. Roth, a labor relations consultant for Local 689 of the Amalgamated Transit Union, said the proposed agreement would provide higher earnings for Metro employes than they would have received under terms of the previous contract. He described the tentative pact as "more expensive" for the transit authority.

Another official, who asked not to be identified, contended that the proposed agreement would ensure savings for Metro. "It costs less than the continuation of the present contract," he said.

In concluding the tentative agreement, the union's negotiators appeared to have accepted substantial cuts in cost-of-living benefits in exchange for sizable reductions in the amount of money employes would have to pay into a pension plan. The effect of these shifts, according to the union, would be to provide Metro employes with more take-home pay.

Under terms of the contract that expired April 30, Metro employes have received pay adjustments every three months to match increases or decreases in the Consumer Price Index. Many local government officials have viewed this provision as generous and costly.

Under the proposed agreement, cost-of-living increases would be paid to Metro employes only if the Consumer Price Index rises by more than 6.5 percent. The cost-of-living payments would amount to 80 percent of any CPI increase above the 6.5 percent level and would be provided only twice, on April 30, 1985, and again the following year.

In addition, Metro employes would receive two general wage increases, each amounting to 6.5 percent. They would take effect on May 1, 1984, and a year later. Metro officials characterized this clause as providing for a "wage freeze" until next May 1.

Metro employes would gain by no longer being required to pay into their pension plan. The payments now amount to 5.5 percent of employes' wages. Under the tentative agreement, the transit agency would bear the full cost of the retirement plan. The union characterized this shift as the equivalent of an 8.3 percent to 9.7 percent rise in employes' wages because it would not be subject to additional income tax or Social Security payments.

Members of Local 689 are scheduled to vote tonight on whether to accept the agreement, which was approved Monday by the union's executive board. The transit authority's board of directors reportedly endorsed the proposed settlement last Friday.

The union represents 5,500 bus drivers, subway operators, mechanics and other employes. If the tentative three-year agreement is ratified, it is expected to set a pattern for a settlement with Teamsters Local 922, which represents another 250 Metro bus employes.

Metro officials have previously described labor negotiations as central to their efforts to hold back the authority's soaring multimillion-dollar deficits. Veteran bus and subway operators now earn $12.04 an hour.

One other key provision would allow Metro to increase its use of part-time bus drivers, a move aimed at cutting transit costs. The Metro system is now restricted to employing part-time drivers on 10 percent of its routes. This ceiling would be increased to 15 percent. In exchange, the union would gain fringe benefits for part-time employes.